1. Field of the Invention
The present invention relates to a manufacturing method of a rare-earth magnet.
2. Description of Related Art
A rare-earth magnet using a rare earth element such as lanthanoid is also called a permanent magnet. The rare-earth magnet using a rare earth element such as lanthanoid is used in a driving motor of a hybrid vehicle, an electric vehicle, and the like, as well as a hard disk and a motor constituting a magnetic resonance imaging device (an MRI device).
In regard to an increase in heat generation amount due to downsizing and high current density of a motor, a demand of heat resistance is more increased relative to the rare-earth magnet to be used. Because of this, how magnetic characteristics of a magnet can be maintained under high-temperature use is one of important research themes in this technical field.
As the rare-earth magnet, general sintered magnets in which crystal grains (a main phase) constituting its structure have a scale of around 3 to 5 μm, and nanocrystalline magnets configured such that crystal grains are fabricated in a nanoscale of around 50 nm to 300 nm have been known. Among the nanocrystalline magnets, a nanocrystalline magnet achieving the above nanofabrication of crystal grains while reducing an additive amount of expensive heavy rare-earth elements and a nanocrystalline magnet using no heavy rare-earth elements are currently attracting attention.
As one example of a manufacturing method of a rare-earth magnet, there has been known such a method that a sintered body is formed by performing pressure molding on a fine powder (magnetic powder) that is obtained by rapidly solidifying Nd—Fe—B molten metal, and hot plastic working is performed to give magnetic anisotropy to the sintered body, thereby manufacturing a rare-earth magnet (oriented magnet). Note that extrusion such as backward extrusion and forward extrusion, upsetting (forging), or the like is applied to the hot plastic working.
Generally, over the whole steps of manufacture and transfer of a magnetic powder, manufacture of a sintered body, and manufacture of a rare-earth magnet, a product to be manufactured in each of the step makes contact with oxygen included in an atmospheric air. As a result, an oxygen concentration inside a structure of the product to be manufactured increases or the product to be manufactured is oxidized, so that magnetic performance of a rare-earth magnet that is finally obtained decreases, which is well known.
As an index of the magnetic performance of the rare-earth magnet, residual magnetization (residual magnetic flux density), a coercive force, and the like are known. For example, it is known that, at the time when hot plastic working is performed, oxygen included in a magnet material breaks a main phase of Nd—Fe—B, thereby reducing a residual magnetic flux density and a coercive force. Further, it is also known that, at the time when grain boundary diffusion of modified alloy occurs to recover a coercive force after hot plastic working is performed, oxygen remaining inside the modified alloy obstructs penetration into the modified alloy. Moreover, it is known that oxygen taken in a magnet reacts with a rare-earth element in a grain boundary phase so as to form an oxide, so that grain boundary phase components effective to divide a main phase magnetically are reduced, thereby resulting in that a coercive force of the rare-earth magnet decreases.
As a technique to reduce an oxygen concentration of a rare-earth magnet, the following related art to prevent contact with oxygen in a manufacturing process of a rare-earth magnet is disclosed.
For example, Japanese Patent Application Publication No. 6-346102 (JP 6-346102 A) and Japanese Patent Application Publication No. 2005-232473 (JP 2005-232473) describe such a technique in which a magnetic powder for a rare-earth magnet is accommodated in a highly-airtight container filled with inert gas, and sintering is performed while the powder is supplied to a mold from the container.
Further, Japanese Patent Application Publication No. 1-248503 (JP 1-248503 A) describes a method for manufacturing a rear-earth magnet in such a manner that a magnetic powder for a rare-earth magnet is filled into a metal can, the can is made airtight under vacuum suction, and hot extrusion press is performed on the can that is heated.
Further, Japanese Patent Application Publication No. 1-171204 (JP 1-171204 A) describes a manufacturing method of a rare-earth magnet in which method a rare-earth magnet ingot is surrounded by a metallic material and then sealed, and hot working is performed on the metallic material thus sealed.
According to the related arts, a concentration of oxygen making contact with the magnetic powder, the sintered body, or the like in a manufacturing process of the rare-earth magnet can be reduced.
However, in the manufacturing methods described in JP 6-346102 A and JP 2005-232473 A, the magnetic powder is filled into the mold from the highly airtight container, so that workability is not good. Accordingly, it takes a long manufacturing time and a cost for the manufacture of the container is required, which may generally increase a manufacturing cost.
Further, in the manufacturing methods of JP 1-248503 A and JP 1-171204 A, hot-press is performed on the metal can or the like. However, for example, a magnetic powder for a Nd—Fe—B rare-earth magnet is a strongly oxidizing material as compared with general metals, so that the magnetic powder inside the metal can or the like is easily oxidized prior to the metal can or the like. Therefore, it is difficult to obtain a high oxidation-suppressant effect with respect to the magnetic powder.